When drying wood in a wood drying kiln, an end-point moisture content of 5% to 20% is normally required. Traditional methods of measuring the moisture content of wood, whilst reasonably accurate towards the end-point moisture content, become less accurate at higher values of the moisture content. At a moisture content of above 30%, the traditional methods become completely unreliable.
For the proper control of the environment in which wood is dried, for example, in a wood-drying kiln, it is important for the moisture content of the wood to be known accurately while the moisture content is still relatively high, e.g. above 30%. If the moisture content is accurately known at these relatively high values it becomes possible to accelerate the drying process considerably, without causing undue stresses in the wood.
The complexity of wood is easily underestimated. Wood is highly an-isotropic both in anatomy and by its electrical and dielectric properties. It is a complex composition of air, water cell-wall structure, organic materials such as cellulose, lignin and resins, inorganic salts. The anatomy is comprised of solid cell-wall structures combined with trachea (tangential hollow tubes) which is either filled with water or air depending on the moisture content (m.c.) of the sample. Furthermore, variations within species are remarkably striking regarding ion-content which translates in conductivity and resistive variations. Species-to-species variations in ion content is even more influential and wide species to species changes in conductivity is experienced.
In order to measure the dielectrics of such a complex medium, the influences of each of these components need be addressed before valuable and usable measurements and methods can be devised. The measurement of the dielectric properties of wood is particularly and unexpectedly troublesome as reported in detail by Torgovnikov[T]. (The citations herein identified by upper case letters are the bibliography at the end of the specification.) Not only is the dielectric highly an-isotropic and grain direction dependent, the unexpected temperature behavior of the conductivity of wood is worth mentioning. It would be expected that wood would have similar characteristics as usual carbon based resistors, which displays a decrease in conductivity with increased temperature (increase in resistance respectively). The conductivity of wood in fact does not follow this trend at all, but rather displays the temperature dependence strikingly similar to a semi-conductor i.e. the conductance increases with increasing temperature. It is clear that if this is not taken into account, measuring methods of, e.g., capacitance of the wood-dielectric will fail at elevated temperatures as large errors will be introduced. This particular fact resulted in several measurement systems to fail in industry for obvious reasons James[R]. To make matters even more troublesome, extremely non-linear anomalies occur regarding the other relevant dielectric components, namely the relative permittivity ∈r also known in layman's terms as the dielectric constant.
Since the relative permittivity gives rise to capacitance via the probe geometry and since capacitance will be what is measured, this influence will be discussed in terms of the capacitance but is equally valid for the relative permittivity. Dielectric constant and as a consequence capacitance increases substantially with increase in temperature compared to more homogeneous dielectrics. However, Torgovnikov [C] cites James's results to display the following anomalies. Not only is the relative permittivity and therefore capacitance wildly frequency dependent, it does so in an unexpected manner. Completely dry (bone-dry) wood has a relative permittivity of 4, while water has a relative permittivity of 80. The relative permittivity of water and bone-dry wood is for all purposes frequency independent except for the normal dispersion variations not of relevance here. However, when water and wood is combined i.e. wet wood is measured, we do not obtain the intuitive combined relative permittivity of 84, but instead values are reported by Torgovnikov and James [C] of ∈r=650000 at certain lower frequencies. This is most certainly an anomaly and to date still unexplained and seemingly not challenged however unlikely it seems. Furthermore the relative permittivity and therefore the capacitance increases dramatically with decrease in frequency compared to minimal change in ∈r detected for pure water and bone-dry wood when not in combination over the same frequency range. In addition the loss-tangent tan δ, which is an indication of how lossy a material is in an applied electromagnetic field, also displays curious anomalies generally not expected from dielectric media. Even the most complex composites usually has a loss-tangent, for which each value of loss-tangent only one value of element of composition can be obtained. With wood as dielectric the loss-tangent generally becomes a relation i.e. the loss-tangent plotted against moisture content is that of a bell-curve Torgovnikov [C] resulting in two moisture contents giving the same loss-tangent reading. This clearly cancels loss-tangent for measurement above f.s.p. in most cases as it results in ambiguity. These complications dwarfs the already significant an-isotropic behavior of ∈r which has different values when the applied electromagnetic field is applied tangential and radially to the wood respectively. The remaining significant behavior of the wood-water relationship is at f.s.p, where free water starts to assemble in the hollow trachea and dissolves salts. These ions then drastically increase the conductivity above f.s.p. to enormous proportions and in effect making any correlation of moisture content above f.s.p. difficult if not impossible. The conductivity of wood therefore becomes an almost constant high value above f.s.p. literally independent of higher moisture contents. The reason for the sudden conductivity increase above f.s.p. is due to the minerals K, P, Al, Fe, Zn, Ca, Mn, Cl, Na and Mg, to name a few which are naturally encountered in wood. The majority of these minerals are dissolved and present in the free water as ions and therefore has a phenomenal influence on conductivity above f.s.p. Below f.s.p. no free water exists and these minerals are then deposited on the cell walls with less influence.
The bounded water (adsorbed water on cell walls) is also changed fundamentally in that the water which is now adsorbed by the cell-walls clearly cannot be rotated easily as a dipole in the applied field. As the wood dries the adsorption to the cell-walls increases giving even more resistance to rotation in the applied electromagnetic field. This results in a curved relationship between ∈r at moisture contents below f.s.p. Above f.s.p. the free water in the hollow trachea are the dominant influence on ∈r and ∈r versus moisture content and the water molecules as dipole can easily and unrestrictedly be oriented in the applied electromagnetic field. This is the reason why ∈r is then linear from f.s.p upwards to 200%. This combined then establishes a curve-linear relationship between ∈r and the moisture content as empirically verified by Skaar[F]. It is therefore evident that two “types” of water exists in the wood-water combination and they influence the dielectric properties in a very different way. The list of behavioral anomalies are not exhausted as there are piezzo electrical effects creating electrical impulses during drying due to crystalline structures in the wood and several more which will not be discussed, although further complications arises due to them. Wood rivals if not champions the most complex composite dielectrics, is rich in anomalies and unexpected behavior. These anomalies and properties are crucial to understand why some measuring processes in prior art, when applied to wood, are irrelevant or non-functional and will be referred to in sequel.
Definition of Terms.                Moisture Content                    Defined as the following percentage, (1)                        
                              M          .          C          .                =                  100          ⁢                                                    M                w                            -                              M                d                                                    M              d                                                          (        1        )                                                where,            Mw=Mass off Wet sample            Md=Mass of Bone-Dry Sample.                        Free water                    Water present in the trachea of the wood sample.                        Bounded Water                    Water chemically bounded to the cell walls of the wood sample.                        Fibre Saturation Point (f.s.p.)                    Wood is comprised of a solid cell-wall structures combined with hollow tubes which is either filled with water or air depending on the moisture content (M.C.) of the sample.            The f.s.p. is the maximum moisture content where all water is absent from the trachea and all the remaining water is chemically bounded to the cell-walls. The typical f.s.p. for softwoods such as Pinus is 30% M.C. For hardwoods, this is typically 40-50% M.C. This, however, varies from species to species according to density.                        Wood drying installation                    A wood drying installation is any construction or device in which wood can be dried, e.g., a kiln.                        Material                    The present invention can be applied to any dielectric material. In case of wood it includes both hard and soft woods, which can be in several states namely dimensional lumber, wood chips, veneer, saw dust or logs.                        
Correlation of Measurement with Moisture Content. Several correlations are possible:                Correlation of the moisture content with the dielectric constant ∈r. This manifests itself in capacitance measurements where capacitance is obtained from ∈r by mans of the probe geometry.        Correlation of the moisture content with the Conductivity. These are mostly referred to as resistance type meters employing alternating or direct current.        Correlation of the moisture content with the loss-tangent (tan δ). These are referred to as loss-tangent measuring principles employing alternating current.        
Dielectric Model of Wood as Medium.
It is well known from literature that the sensitivity of inductance to moisture content of wood is negligible. The dielectric for wood would then comprise of the various influences of dielectric constant and conductivity σ only.
The full dielectric model for wood is displayed in FIG. 1. All the different kinds of polarizations evident for wood are represented by the various capacitances. They are Ce Ca Cd Cv and Cz, effected by electronic, ionic, dipole, interfacial, electrolytic, polarizations. Rd, Rv, Rz and R1 are the resistances resulting from energy losses at dipole, interfacial, electrolytic, and resistance related to the direct current, respectively.
The model in FIG. 1 is for analytical purposes and a practical model used in determination of dielectric properties of wood for commercial systems is the Thevenin-Norton, lumped model as in FIG. 2 where the representative dielectric components are now the lumped values Cx and Rx.During discussion in sequel, it will be understood that the model as in FIG. 2 is used.
Definition of and Comparisons Between Resistive and Capacitive Sensors.
There are several other methods available such as “Neutron Probes” and “Infra Red” to name a few. These methods are undesirable for several reasons. It is of importance to focus on the two dielectric measurement principles namely “Capacitance” and “Conductivity” of the wood sample as is clear from. A serious concern regarding the use of the type of measurement in literature is now raised as it presents considerable confusion if not addressed.                It is obvious from that a measurement principle which claims to be a Capacitance meter must be able to single out and measure only the Cx in FIG. 2 and be generally insensitive to changes in Rx.        Likewise, a measurement principle which claims to be a Resistance meter must be able to single out and measure only the Rx in FIG. 2 and be generally insensitive to changes in Cx.        Then, for a measurement principle to claim to be a loss-tangent meter (tan δ) it must be clear that the meter combines the Rx and Cx components in such a way as to represent loss-tangent closely.        Any measurement principle unable to separate the components Rx and Cx in 2 will therefore be a non-linear convolution of dielectric properties and no fundamental information regarding Rx and Cx can be extracted. The output of such measurement is therefore some convoluted indication of the influences of both Rx and Cx. This measurement type will be referred to a of type “convoluted” in sequel.        Measurement methods which can measure and identify Rx, Cx and tan δ accurately and independently will be described as “True-measurements” in sequel.        Furthermore, if a single measurement principle can obtain all the separate dielectric properties at once and in real time, it will be called “real-time measurements” in sequel. The patent applications [E] and [N], essentially presents a real-time form of these quantities.        
The present invention allows the use of a general impedance as a detector, opening up a method to use a cost-effective detector to measure at both high and low frequencies, using combinations of resistance, capacitance, and inductance.
In order to measure at high frequencies, it is necessary to keep probe wires as short as possible and interference at the minimum. The present invention approaches this by using the kiln as a co-axial cable to deliver both power and data in the closest proximity of the measuring device containing the detector. An example of employing the kiln as a co-axial cable is the measurement of the fiber saturation point. Further advantages of using a kiln as a co-axial cable is better reliability of the measurement system, because the use of multiple probe wires suspended from the kiln walls to the probes in the wood can be eliminated, on which wood often could fall and disable the system. Moreover, the installation of the measuring system is much simplified by not requiring the internal wiring, which can take up to two days of installation time by a contractor.
In comparison to the present invention, Venter et al. [N] uses a series resistance in combination with a dielectric model as an algorithm in order to obtain the components Rx, Cx of the impedance Zx independently and simultaneously. The current invention does not use a series resistance, but a general impedance Zs which is not a single resistance Rx. Steele [H] does not disclose a detection circuit and is therefore not enabling a measurement method. Steele requires the detection circuit to be an oscilloscope, which by itself is insufficient, as a detection circuit is needed in conjunction thereof. Such a detection circuit, is e.g. a series resistor. Inclusion of such a series circuit would lead to the invention as disclosed in [N].
Logan [?] discloses a back-to-back matched transistor circuit of which one half-circuit measures moisture content of the wood while the other half-circuit measures a reference load where the reference load is a capacitance. The reactive load is not driven through an impedance as disclosed in the current application, but compared with a fixed capacitive reference. They are therefore not equivalent methods.